问题 选择题

以下实例中,力对物体做功的是(  )

A.小明将水桶从地面上提起

B.吊车吊着货物在空中水平移动

C.用力搬桌子而未搬起

D.举重运动员举着杠铃不动

答案

A、将水桶从地面上提起,是在力的方向上移动一段距离,对物体做功了,符合题意;

B、虽然有力也有距离,但此距离不是在力的方向上,力与距离垂直,没有做功,不符合题意;

C、搬桌子而未搬起,有力而无距离,没有做功,不符合题意;

D、举重运动员举着杠铃不动,这样是有力但无距离,没有做功,不符合题意;

故选A.

解答题
阅读理解

       If your mother wants to tell you something, she uses words. Birds cannot talk as we do. But some birds can make sounds to warn their young of danger. They have their own way to make the young birds do certain things.

The jackdaw is a kind of blackbirds that lives in Europe. Jackdaws live together in flocks(群).Young jackdaws do not know their enemies. When an older jackdaw sees a dog, it makes a loud rattling(格格响的)sound. The younger birds know this sound means an enemy is nearby. The sound warns them to know their enemies.

If a young jackdaw is in a dangerous place, a jackdaw parent flies over him from behind. The parent bird flies low over the young bird’s back. The parent’s tail feathers(羽毛) move quickly from side to side. It is trying to say,“Follow me.”At the same time, the parent calls out,“Key-aw,”key-aw.”The parent means,“Fly home with me.”The young bird then follows the older one home. Young jackdaws do not have to learn what certain sounds mean. They know the meaning of these sounds form the time they hatch(孵出).

72.The jackdaw lives in       .

A.Europe     B.Australia   C.America   D.Africa

73.Which of the following does this story lead you to believe?

A.All animal parents can talk to their young.   

B.Dogs are the most dangerous enemies for jackdaws.

C.Young jackdaws know the meaning of their parents’ sound when they grow older.

D.Some birds can give certain information to one another.

74.Parent jackdaw can use their tail feathers to       .

A.ask their young to follow them      B.play a game with the young

C.tell the meal time            D.give a warning of a fire

75.This story tells much about       .

A.the danger of jackdaws   

B.ways in which mother can talk to their children

C.the way jackdaws warn their young of danger

D.how the jackdaw are living

76.When an old jackdaw sees a dog, it       .

A.calls out,“Follow me.”       B.makes a loud sound

C.flies away               D.fights the dog